The co-activator DCoH (dimerization cofactor of HNF-1) interacts with the 32 amino acid dimerization domain of the transcription factor HNF-1. The structure of this complex demonstrates that HNF-1 interacts with the helical surface of DCoH, and confirms that the dimerization domain of HNF-1 forms a 4-helix bundle. The experimental phases obtained from a MAD experiment with seleno-methionine substituted DCoH has not been good enough to build one of the HNF-1 helices and the N-terminal helices of one of the DCoH dimers. We propose to improve our experimental phases by conducting a MAD experiment on a seleno-cysteine substituted peptide, in addition to the seleno-methionine DCoH.